Taxonomic And Phylogenetic Relationships Between Citrus And Its Close Relatives Based On CpDNA Sequences

The genus Citrus established in 1753 by Carl Linnaeus belongs to the subfamily Aurantioideae of the family Rutaceae. In the past more than 250 years, a series of studies from morphology to molecular markers on the taxonomy and evolution of the genus Citrus has been carried out both in China and abroad. However, the current views about the subgenus subdivision and the number of species in Citrus are still unclear. The phylogenetic framework of Citrus (the number of wild primitive species, their origins and interspecific relationships) has been unestablished. Swingle (1943) classified the genus Citrus and its five close relatives, Poncirus, Fortunella, Microcitrus, Eremocitrus and Clymenia into the true citrus fruits group based mainly on the number of stamen and structure of pulp-vesicles of the genera. Because of bud mutation, long cultivation history and hybrid compatibility, the taxonomic and evolutionary relationships between Citrus and its close relatives have remained unresolved. Citrus is one of the world's most important fruit crops, a better understanding of the phylogenetic relationship between Citrus and its closely related genera is needed for futrue variety breeding, germplasm conservation and utilization.The angiosperm chloroplast genome has usually a small size (135 to 160 kb) and simple structure. Its predominant maternal inheritance and the characteristics of being inherited as a single linkage group (a single-copy gene) of all cpDNA sequences make them an especially useful tool in plant systematics study. In the present study we analyzed three cpDNA regions (TrnL - TrnF, PsbH - PetB and TrnS - TrnG) of thirty representative genotypes of the true citrus fruit trees group. Using PAUP* version 4.0bl0 software we constructed the maximum parsimony (MP) tree of the materials analyzed with Severinia, Atalantia, Murraya as outgroups. Our strict consensus tree suggested that the true citrus fruit trees group is a monophyletic group with 100% statistical support. Within the group, four strongly supported clades (A-D), can be clearly identified: (1) clade A consists of all the three accessions from the genus Poncirus with 100% statistical support; (2) clade B composes of all five accessions of the genus Fortunella with 100% statistical support; (3) Clade C formed by M. australis, E. glauca, Cl. Polyandra, C.medica and C.medica. var sarcodactylis with 100% statistical support; and (4) clade D contains all other Citrus genotypes and M. australasica with 97% statistical support. Finally, the phylogenetic relationships between Citrus and its close relatives were discussed in details.